1. Field of the Invention
The present invention generally relates to a light emitting element employing porous silicon and an optical device utilizing the light emitting element. More specifically, the present invention is directed to a charge injection type light emitting element (LED) having a pn junction employing porous silicon, and also to an optical device such as an opto-optical converting element, a light transmitting element, a photocoupling circuit element, a photocoupling element and a display device, which contain, in principle, such a light emitting element.
2. Description of the Related Art
Conventionally, it is considered that since a silicon semiconductor corresponds to an indirect interband transition semiconductor, a light emitting element could not be manufactured by employing such a silicon semiconductor. As a consequence, the conventional light emitting elements with employment of the pn junction have been manufactured by the III-V group compound semiconductor, II-VI group compound semiconductor, and IV-VI group compound semiconductor. However, there are many materials to manufacture a silicon semiconductor. Silicon semiconductors having large areas can be produced at low cost, and the technique to manufacture single crystal could be established, as compared with those of the compound semiconductor. There are various merits in the silicon semiconductors that logic elements, calculation elements, drive elements, and light receiving elements and the like with high reliability can be assembled on a single substrate at a high integration degree, because of high device designing/manufacturing techniques, which could not be practically realized with the compound semiconductor. Accordingly, strong demands have been made to obtain a light emitting element with employment of silicon, especially a charge injection type light emitting element employing a pn junction, capable of being applied to a laser at a final goal. To the contrary, L. T. Canham reported in 1990 that the porous silicon fabricated by electrochemical anodizing single crystal silicon into a hydrogen fluoride solution, indicates strong photoluminescence at a room temperature (see Applied Physics Letters 57, (10), Sep. 3, 1990, pages 1046-1048). This report implies that a light emitting element may be manufactured by silicon. Thereafter, many researches have been carried out to analyze a generation mechanism of this photoluminescence. However, since no one could find such a material by which a pn junction can be formed with porous silicon at a better characteristic and a light emitting diode (LED) can be manufactured with this pn junction, a charge injection type light emitting element in a pn junction mode with this porous silicon could not be realized.
On the other hand, there are various optical devices such as an opto-optical converting element, a light transmitting element, a photocoupling circuit element, and a photocoupling element. For instance, as disclosed in a Japanese publication "Physics on Optical/Electronic Integrated Circuit" by H. Matsueda, pages 279-309, the plane type light transmitting element is known that the optical transmission unit containing the multilayer light emitting element of GaAs/AlGaAs, and also an FET functioning as a switching modulation element for modulating the light signal sent out from the optical transmission unit in response to an input electric signal are fabricated on a compound semiconductor substrate such as a GaAs substrate and an InP substrate. Also, there has been proposed in "Parallel Process System realized by 3-Dimensional Shared Memory" in Electronics, October 1981, that the light emitting element with employment of the compound semiconductor such as GaAs and the light receiving element are positioned opposite to each other on a single wafer, as the 3-dimensional photocoupling circuit element for performing the data communication among the circuit elements such as memories in the computer by utilizing the light signals.
However, even in the above-described conventional optical devices, since the expensive compound semiconductor substrate is employed, and also the light emitting element is fabricated with the compound semiconductor, the manufacturing cost becomes high and large integration becomes difficult, resulting problems. Therefore, an optical device with employment of a light emitting element made by silicon has been demanded.
Many attempts have been made in that the band structures of the silicon semiconductor are varied by utilizing the quantum effect to emit light. These silicon semiconductors capable of emitting the light have been manufactured by way of the molecular beam epitaxicy (Applied Physics Letter 56, (4), Jan. 22, 1990, pages 340-342), the plasma etching method (Applied Physics Letter 59, 1991, pages 1603-1605), and the RF sputtering method (Physical Review B 38, Sep. 15, 1988, pages 5726-5729). However, since very huge installations are required and very difficult manufacturing techniques are needed, this technique could not practically utilized.